Among known underwater respiration devices are those that supply air via a conduit from the Earth's atmosphere to a submerged user or, in the case of SCUBA, comprise a portable tank with breathable compressed gases including oxygen. In open-circuit SCUBA systems, the breathed, exhaust gas is discarded in the form of bubbles with each breath. Closed-circuit systems recycle the exhaust gas by adding oxygen to and removing carbon dioxide from exhaled breaths.
U.S. Pat. No. 3,333,583 discloses a closed-circuit underwater respiration device which purifies and recycles a diver's exhaled breath. This purification is achieved by driving the exhaust breath through gas permeable tubes, which are surrounded by a current of seawater. Oxygen dissolved in the seawater then passively diffuses across the tubes into the exhaled breath while carbon dioxide similarly diffuses out. The breath is then supplied to the diver for breathing and the process is repeated indefinitely.
U.S. Pat. No. 3,656,276 discloses a closed-circuit method and apparatus for reoxygenating and removing carbon dioxide from stale, breathed air in an underwater habitat by mixing it with seawater in intimate and agitated contact, and subsequently separating the refreshed air from the seawater.